Remove dependence on antigrain.com for the examples
Some minor fixes requested by users
M_SQRT1_2 is from the POSIX standard, not any C or C++ standard. I can update the file with a more precise version of the constant.
Fix off-by-one error in mol_view example
Update library revision from 2.6.0 to 2.7.0
Replicate change from r57 made in pixfmt_alpha_blend_rgba to pixfmt_custom_blend_rgba
Update version to 2.6.0. Use modern macros for configure.ac
Replace all C standard headers with C++ equivalent headers, use std:: namespace everywhere (except examples)